This invention relates to steering systems for vehicles. More particularly, this invention relates to a mechanical back-up system for a steer-by-wire system for vehicles and a security system therefor.
Automobiles are conventionally equipped with a pair of front road wheels that are steered to enable the vehicle to turn left and right as it maneuvers on land. It is also known to provide actuators for steering rear wheels in automobiles. In the past, vehicle steering systems commonly employed a mechanical connection between the driver-operated hand wheel and the front road wheels of an automotive vehicle. As the driver rotated the hand wheel, a mechanical linkage through the vehicle""s tie-rods actuated the road wheels sometimes with the assistance of a power assist steering motor or hydraulic piston.
Recently, steer-by-wire steering systems have been introduced into automotive vehicles to provide road wheel steering function. Included in a typical steer-by-wire steering system is a hand wheel actuator for monitoring the angular position of the steering wheel, and road wheel motor actuators which are controlled by controllers in response to tracking the sensed angular displacement of the hand wheel from a central position. In contrast to prior steering systems, the steer-by-wire steering system does not employ a mechanical linkage between the steering wheel and the individual road wheels. Exemplary of such known steer-by-wire systems is commonly-assigned U.S. Pat. No. 6,176,341, issued Jan. 23, 20001 to Ansari. Other proposed systems are described in xe2x80x9cFuture Electrical Steering Systems: Realizations with Safety Requirementsxe2x80x9d by Harter, et al. and xe2x80x9cSteer-by-Wirexe2x80x9d by Ebner.
Because there is no mechanical linkage between the hand wheel and road wheels in a steer-by-wire system, steering enhancements such as variable-ratio steering and oversteer/understeer correction are easily implemented to improve the drivability and safety of the vehicle. At the same time, mechanical back-up systems are known to engage a traditional mechanical steering linkage in the event of a failure of the electronic steer-by-wire system. See, for example, xe2x80x9cSteer-by-Wirexe2x80x9d by Dr. Christian Ebner, for a typical example of such a system, which includes a hand wheel actuator having a steering shaft, and a steering column selectively engaging the shaft via a clutch, which is electrically actuated. It would be desirable to provide a steer-by-wire system with a mechanical back-up device that does not require moving parts to engage, thereby improving overall reliability.
At the same time, mechanical column locks, used as a theft deterrent and to prevent the steering wheel from turning as an aid when exiting and entering the vehicle (thereby providing a grab-handle) presents the problem in that its convenient location presents thieves with the opportunity to disable it, and adds undesirable weight at the top of the steering column. Extra weight near the top of the steering column requires a strong support and presents safety issues during a frontal collision.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a steering system comprising a hand wheel positioned for manipulation by a driver and a steering shaft, extending from the hand wheel and a magnetic column lock. The steering shaft is supported by a steering column housing so that it can rotate on its own axis. The magnetic column lock includes a rotor attached to steering shaft, a stator attached to column housing, and magnetorheological fluid filling a space between the rotor and stator.
In another aspect of the steering system, the steering shaft extends from the hand wheel through a hand wheel actuator having a position sensor and a motor for providing driver feedback. A steering column extends from a lower portion of said steering shaft to a rack, and includes a pinion at its lower end for engagement with the rack, so that the rack translates left and right when the pinion is rotated in corresponding directions. The rack is connected to the front road wheels and causes the front road wheels to steer when it is translated. An electronic controller receives signals indicative of an angular position of the hand wheel from the position sensor, and a road wheel actuator connected to the steering rack receives signals from the electronic controller and repositions the steering rack in response to the signals from said electronic controller. A magnetic clutch having MR fluid is provided for selectively engaging said steering shaft to said steering column.